1. Field of the invention
The present invention relates to an electron gun for a color TV or industrial high definition cathode ray tube, and more particularly, to a focusing electrode in an electron gun for a color cathode ray tube, which can provide more freedom in electron gun design and reduce errors during the assembly of the electron gun.
2. Discussion of the Related Art
The electron gun in a color cathode ray tube focuses three electron beams emitted from cathodes onto a surface of red, green and blue fluorescent materials coated inside of a cathode ray tube so that each of the fluorescent materials reacts to the electron beams to luminesce, to form a pixel on a screen.
FIG. 1 illustrates a sectional view of a general color cathode ray tube.
Referring to FIG. 1, the color cathode ray tube 4 includes an in-line type electron gun 2, deflection yokes 3 for deflecting electron beams 1 in the up and down and the left and right directions and a screen 5 for forming pixels in reaction to the electron beams 1. The screen includes an inside surface 6 coated with fluorescent materials, a funnel 7 that converges from the rim of the screen 5 toward the rear of the tube 4 and a neck part 8 formed at an end of the funnel 7. The in-line type electron gun 2 is mounted inside of the neck part 8 and the deflection yokes 3 are mounted outside of the neck part 8. A shadow mask 9, having a plurality of electron beam pass-through holes 91 for allowing selective collision of the electron beams 1 shot from the in-line type electron gun onto the fluorescent surface 6, is provided between the fluorescent surface 6 and the electron gun 2.
FIG. 2 illustrates a cross sectional view of the in-line type electron gun shown in FIG. 1, FIG. 3A illustrates examples of distortion of electron beam spots on a screen caused by a non-uniform magnetic field formed by deflection yokes, and FIG. 3B illustrates examples of correction of the electron beam spots shown in FIG. 3B by a dynamic quadrupole lens formed by a focusing electrode having burring parts.
Referring to FIG. 2, the in-line type electron gun 2 generally includes a tripolar part 21 and a main focal electrostatic lens part 22. The tripolar part 21 includes, cathodes 23 for emitting thermal electrons following heating by heaters 231 provided therein, a controlling electrode 24 for controlling the thermal electrons, and an accelerating electrode 25 for accelerating the thermal electrons. The main focal electrostatic lens part 22 disposed next to the tripolar part 21 includes a focusing lens 26 and an anode 27. Predetermined voltages are applied to the electrodes; in general, the controlling electrode 24 is grounded, the accelerating electrode 25 receives of a low voltage of 500.about.1000 V, the anode 27 receives of a high voltage of 25.about.35 Kv, and the focusing electrode 26 is applied of an intermediate voltage, a voltage corresponding to 20.about.30% of the voltage applied to the anode 27.
The operation of the in-line type electron gun for a color cathode ray tube having the aforementioned system will be explained.
Upon application of predetermined voltages to the electrodes, voltage differences are formed between the electrodes so that the electron beams emitted from the cathodes are controlled and accelerated to a predetermined intensities by the controlling electrode 24 and the accelerating electrode 25. A voltage difference formed between the focusing lens 26 and the anode 27 forms equipotential planes, which, collectively, act as the main focal electrostatic lens. Accordingly, the electron beams, accelerated by the voltage difference of the anode 27 toward the screen, are focused by the main focal electrostatic lens, pass through the electron beam pass-through hole in the shadow mask 9, and collide on the fluorescent surface 6 on the central part of the screen, to form a pixel. While the focusing of the electron beams 1 onto the central part of the screen is made possible by the main focal electrostatic lens, the deflection of the electron beams 1 by the deflection yokes 3 is required for the sequential scanning of the electron beams onto each region of the screen. There is mismatch of the convergence in the deflection of the electron beams by means of the deflection yokes due to the in-line configuration of the electron gun and the difference of curvatures in the screen. The mismatch of the convergence can be corrected by providing a self convergence of the beams using deflection yokes which can form a non-uniform magnetic field. However, the application of the non-uniform magnetic field causes a problem in which the electron beam forms a horizontally elongated spot with a haze, which is a thin dispersion of an image, on the upper and lower sides of the spot, as shown in FIG. 3A. In order to solve the problem, a dynamic quadrupole lens which is operative synchronous to a deflection synchronizing signal has been used for correction of an astigmatism when the electron beam is deflected toward periphery of the screen.
FIG. 4A illustrates a perspective view of a two-part focusing electrode assembly for a conventional in-line type electron gun, which can form the dynamic quadrupole lens.
Referring to FIG. 4A, the focusing electrode 26 includes a first focusing electrode 261 to which a constant voltage is applied, a second focusing electrode 262 arranged next to the first focusing lens to which a dynamic voltage is applied to make a voltage difference of about 300 V.about.1000 V depending on the extent of deflection of the electron beam, oppositely faced surfaces 265 and 266 of the first and second focusing electrodes 261 and 262 at one ends thereof each having first and second electron beam pass-through holes(263c, 263s and 264c, 264s), and a pair of burring parts 267c and 267s at upper and lower portions of the circumference of each of the electron beam pass-through holes 264c and 264s in the second focusing electrode. When the first and second focusing electrodes are in place, each of the burring parts 267c and 267s are inserted in the electron beam pass-through holes 263c and 263s in the first focusing electrode.
As explained, a dynamic quadrupole lens is formed by the voltage difference between the first focusing electrode 261 to which a low static voltage is applied and the second focusing electrode 262 to which a high dynamic voltage is applied. Particularly, due to the burring parts 267c and 267s provided on upper and lower parts of the electron beam pass-through holes 263c and 264s in the second focusing electrode 262 which diverges the electron beam, the diverging power acts stronger than the converging power from the first focusing electrode 261 which converges the electron beam to correct the electron beam into a vertically elongated form. Accordingly, the horizontally elongated form of astigmatism of the electron beam caused by the deflection yokes can be corrected as shown in FIG. 3B.
However, despite the aforementioned advantage of astigmatism correction capability of the conventional two-part focusing electrode in application to an electron gun, there have been problems which actually impede application of the focusing electrode to the in-line type electron gun.
First, the voltage difference of about 300 V.about.1000 V between the voltages applied to the first and second focusing electrodes 261 and 262 might damage parts of the electron gun in case of the occurrence of discharge between them, which causes a problem of shortening the life time of the cathode ray tube. In order to prevent such an occurrence of discharge, as shown in FIG. 4B, the in-line type electron gun under production currently has been designed to have a pitch S, which is a distance between adjacent axes of the electron beam pass-through holes 263c, 263s and 264c, 264s, of 5.5 mm, a diameter D2 of each of the electron beam pass-through holes 264c and 264s in the second focusing electrode of 4.0 mm, a thickness t of each of the parts of the electron gun of 0.33 mm, a bridge width, which is a distance between adjacent electron beam pass-through holes 263c and 263s in the first focusing electrode of b mm, and a gap between the electron beam pass-through holes 263c and 263s in the first focusing electrode and the burring parts 267 limited to (a&gt;0.2) which does not cause discharge. However, if an electron gun is designed to have the aforementioned dimensions, since the diameter D1 of each of the electron beam pass-through holes 263c and 263s in the first focusing electrode should be 5.06=(4 mm+0.33 mm.times.2+0.2 mm.times.2) at the minimum, only 0.46 mm remains for the bridge width b. This causes deformation of the bridge b from the heat applied to the bridge during fusion of bead glass(not shown) in an assembly of the electron gun. Even in the case when a burring part 268c and 268s (shown in dotted lines in FIG. 4B), which is inserted into the second focusing electrode, is provided around each of the electron beam pass-through holes 263c and 263s in the first focusing electrode for prevention of the deformation of the bridges, there has been a conflict that a 0.66 mm burring part is needed on the bridge of 0.46 mm width, because two burring parts should be formed between two adjacent electron beam pass-through holes 263c and 263s in the first focusing electrode, i.e., on both rims of the bridge b(dotted lines in FIG. 4B). Thus, formation of the burring parts 268 at the electron beam pass-through holes 263c and 263s in the first focusing electrode is not possible.
Secondly, in assembly of the electron gun, after mandrels are inserted from the control electrode up to the anode through each of the electron beam pass-through holes in each of the electrodes to fix the electrodes thereto, preventing the electrodes from shaking, one pair of bead glasses are fusion welded on both sides of the electrodes to complete assembly of the electron gun. However, since the mandrel has an outside diameter tightly fit to the inside diameter of the second focusing electrodes 264c and 264s, and the diameter of the electron beam pass-through holes 263c and 263s in the first focusing electrode is greater than the electron beam pass-through holes 264c and 264s in the second focusing electrode, the first focusing electrodes could not be fixed to the mandrel firmly, resulting in movement of the first focusing electrode 261 during the bead glass fusion welding, which causes misassembly of the first focusing electrode 261 so that the electron gun can not provide its designed performance.
Thirdly, the magnetic field from the dynamic quadrupole lens weakens a focusing power of the main focal electrostatic lens to the outer electron beams, which deteriorates the resolution.